The study of fatigue in materials calls for stressing the material in question in a manner similar to the conditions encountered in its operating environment. Fatigue resulting from the stress occurs as surface changes such as grain interface deformations, surface microcrack initiation, and the growth of small cracks.
Cyclic stresses affect different materials in different ways, particularly in the presence of other operating environmental conditions such as temperature extremes. High-performance materials, such as metal alloy matrix materials or metal-ceramic matrix materials, tend to have good resistance to cyclic fatigue as well as to high temperatures and high loading stresses. However, the complexity of such materials requires that their performance characteristics be studied experimentally to establish their practical design and fatigue endurance limits.
Conventional equipment for subjecting a material to cyclic stress has provided a vibration frequency (cyclic stress rate) that is either too low or too high (ultrasonic). In the case of a stress rate that is too low, excessive testing time is required to accumulate a desired number of vibration cycles, which may be as many as several million or a billion vibration cycles. In the case of a stress rate that is too high, the result is unrepresentative physical effects in the materials, such as excessive internal friction and self-heating. For example, present-day stress-strain testing machines are not capable of cyclic stress rates above about 100 Hz, and ultrasonic tests are typically performed at 20,000 Hz. However, turbine engine blades experience vibration rates in the range of 1,000 to 4,000 Hz. Thus, cyclic fatigue effects in most materials have not been studied at cyclic stress rates that are characteristic of the application for which the material is used.
Furthermore, a complete study of fatigue effects in materials requires that fatigue-causing dynamic stresses be simultaneously superimposed on static loading conditions. This capability is practical in present-day low-frequency machines but has not been implemented in ultrasonic machines.
Also, cyclic fatigue is a progressive mode of failure in many materials, requiring that the surface of the test specimen be examined periodically to observe incremental changes. In this regard, there is experimental evidence that intermittent sequences of cyclic loading interspersed with low-load static quiescence, as well as the time duration of each of the loading and quiescent states, has an influence on the fatigue-induced defects and the fatigue life of many materials. Therefore, examination of a test specimen immediately after a cyclic stress sequence (and, ultimately, during cyclic loading) is an important requirement in fully understanding the characteristics of material fatigue. Conventional testing machines are not designed to be integrated with specimen inspection systems such as a scanning electron microscope (SEM) or a high magnification metallurgical microscope. As a consequence, the stress-inducing process and the specimen-inspecting process are separated. This separation could cause important fatigue effects to be missed because of the time required to dismount and install the specimens in different test setups.